1. Field of the Invention
The present invention relates to a method of controlling an oscillatory friction-reducing maneuver of a valve element journaled in a bearing for movement to regulate a degree of opening of the valve, and to a valve assembly having an element operable according to the method.
2. Description of the Prior Art
In valves for which the degree of opening is automatically regulated with a mechanically journaled element friction in the bearing can cause problems. One of the most common is that frictional forces cause the element to stick in a particular position so that a change in the force applied to the element in order to move it to another position can have unpredictable results. Moreover, the magnitude of the frictional force itself is often unpredictable, since it may change with the age of the valve, the pressure of the element against the bearing etc., so that the force which must be applied to the element in order to overcome the frictional force can be unpredictable.
It is known from U.S. Pat. No. 5,787,924 to provide, in a valve having a mechanically journaled element as described above, a controller which exerts a force on the element to move it to a desired position and then exerts an additional oscillatory force of a predetermined maximum magnitude, a working amplitude, to cause sympathetic oscillation of the element about the desired position by amounts sufficiently small so as not to adversely effect the flow control operation of the valve. This continuous oscillatory motion is designed to prevent the element sticking in the journal arrangement. The working amplitude of this additional force may even be made to be dependent, in a known manner, on the magnitude of an error signal representing the known position of the element and the desired position.
However, the magnitude of the working amplitude of the additional force, or its dependency, is factory pre-set and is typically the same for all valves of a particular type. If set too low it may not overcome the frictional forces which could vary, particularly as the valve ages. If set too high then unacceptably large oscillations in the position of the element may result. A compromise magnitude or dependency must therefore be pre-set which ensures that, for all valves, the valve will operate properly under a large cross-section of conditions.
An object of the present invention is to provide a method and an electromagnetic valve operable according to the method in which the working amplitude of the additional force causing the oscillations is adapted to the operating conditions of the valve as well as to the mechanical properties of an individual valve.
In accordance with the present invention this object is achieved in a method for controlling a friction reducing maneuver of the valve element whereby variations in the magnitude of a property, such as flow or pressure of fluid flowing through the valve, which is affected by a degree of opening of the valve and so is indicative of the degree of movement of the journaled element, is monitored as the maximum amplitude of the force causing the element to perform the friction reducing maneuver is varied. The maximum amplitude of the force is then set to that determined from the monitored variations as providing the minimum variation. Thus the amplitude of the force causing the oscillations need not be pre-set but is made dependent on the prevailing conditions within the valve. This ensures that only sufficient additional force is applied to cause the element to oscillate while minimizing the adverse effects on the fluid passing through the valve.
Preferably the method includes the step of preventing movement of the journaled element in directions other than that required to establish a degree of opening of the valve. This controls a source of variation in friction and so reduces the number of occasions during the operation of the valve for which the amplitude for minimum variation needs to be determined. Indeed to may be sufficient to carry out this determination only once, for example before delivery, during the lifetime of the valve.
The aforementioned object also is achieved in accordance with the present invention in a valve, such as an electromagnetic valve, operable according to the above-described method of the present invention and so possessing the advantages associated with that method.